The present invention relates generally to methods for the direct processing of metal passing through an electroslag refining operation. More specifically, it relates to methods for atomizing or otherwise directly processing a stream of metal which stream is generated directly beneath an electroslag processing apparatus. Most specifically, it relates to methods for the reprocessing of the solidified metal, overspray powder, produced during a spray forming process but which did not become attached to the preform.
It is known that processing relatively large bodies of metal, such as superalloys, is accompanied by many problems which derive from the bulky volume of the body of metal itself. Such processing involves problems of sequential heating and forming and cooling and reheating of the large bodies of the order of 5,000 to 35,000 pounds or more in order to control grain size and other microstructure. Such problems also involve segregation of the ingredients of alloys in large metal bodies as processing by melting and similar operations is carried out. In the past, a sequence of processing operations was sometimes selected in order to overcome the difficulties which arise through the use of bulk processing and refining operations.
One such past sequence of steps involved a sequence of vacuum induction melting followed by electroslag refining and followed, in turn, by vacuum arc refining and followed, again in turn, by mechanical working through forging and drawing types of operations. While the metal produced by such a sequence of steps was highly useful and the metal product itself is quite valuable, the processing through the several steps was expensive and time-consuming.
For example, the vacuum induction melting of scrap metal into a large body of metal of 20,000 to 35,000 pounds or more can be very useful in recovery of the scrap material. The scrap may be combined with virgin metal to achieve a nominal alloy composition desired and also to render the processing economically sound. The size range is important for scrap remelting economics. According to this process, the scrap and other metal is processed through the vacuum induction melting steps so that a large ingot is formed and this ingot has considerably more value than the scrap and other material used in forming the ingot. Following this conventional processing, the large ingot product is usually found to contain one or more of three types of defects and specifically voids, non-metallic inclusions and macrosegregation.
This recovery of scrap into an ingot was the first step in a refining process which involves several sequential processing steps. Some of these steps were included in the subsequent processing specifically to cure the defects generated during the prior processing. For example, such a large ingot may then be processed through an electroslag refining step to remove a significant portion of the oxide and sulfide inclusions which may be present in the ingot.
Electroslag refining is a well-known process which has been used industrially for a number of years. Such a process is described, for example, on pages 82-84 of a text on metal processing entitled "Superalloys, Supercomposites, and Superceramics". This book is edited by John K. Tien and Thomas Caulfield and is published by Academic Press, Inc. of Harcourt Brace Jovanovich, and bears the copyright of 1989. The use of this electroslag refining process is responsible for removal of oxide, sulfide and other impurities from the vacuum induction melted ingot so that the product of the processing has lower concentrations of these impurities. The product of the electroslag refining is also largely free of voids and non-metallic inclusions.
However, a problem arose in the electroslag refining process because of the formation of a relatively deep melt pool as the process is carried out. The deep melt pool, which has a relatively slow solidification rate, resulted in a degree of ingredient macrosegregation and in a less desirable microstructure. Defects produced by macrosegregation were visually apparent and were called "freckles". One way to reduce freckles was by reducing the diameter of the formed ingot but such reduction could also adversely affect economics of the processing.
To overcome this deep melt pool problem, a subsequent processing operation was employed in combination with the electroslag refining, particularly to reduce the depth of the melt pool and the segregation and microstructure problems which result from the deeper pool. This latter processing was a vacuum arc refining and it was also carried out by a conventional and well-known processing technique.
The vacuum am refining started with the ingot produced by the electroslag refining and processes the metal through the vacuum arc steps to produce a relatively shallow melt pool and to produce better microstructure, and possibly a lower nitrogen content, as a result. Again, for reasons of economic processing, a relatively large ingot of the order of 10 to 40 tons was processed through the electroslag refining and then through the vacuum arc refining. However, the large ingots of this processing has a large grain size and may contain defects called "dirty" white spots.
Following the vacuum arc refining, the ingot of this processing was then mechanically worked to yield a metal stock which has better microstructure. Such a mechanical working may, for example, involve a combination of steps of forging and drawing to lead to a relatively smaller grain size. The thermomechanical processing of such a large ingot requires a large space on a factory floor and requires large and expensive equipment as well as large and costly energy input.
As was indicated above in describing the background of this art, one of the problems was that one processing step results in some deficiency in the product of that step so that another processing step was combined with the first in order to overcome the deficiency of the initial or earlier step in the processing. However, when the necessary combination of steps was employed, a successful and beneficial product with a desirable microstructure was produced. The drawback of the use of this recited combination of processing steps was that very extensive and expensive equipment was needed in order to carry out the sequence of processing steps and further a great deal of processing time and heating and cooling energy was employed in order to carry out each of the processing steps and to go from one step to the next step of the sequence as set forth above.
The processing as described above has been employed in the application of superalloys such as IN-718 and Rene 95. For some alloys the sequence of steps has led to successful production of alloy billets, the composition and crystal structure of which are within specifications so that the alloys can be used as produced. For other superalloys, and specifically for the Rene 95 alloy, it was usual for metal processors to complete the sequence of operations leading to specification material by adding the processing through powder metallurgy techniques. Where such powder metallurgical techniques were employed, the first steps in completing the sequence are the melting of the alloy and gas atomization of the melt. This was followed by screening the powder which was produced by the atomization. The selected fraction of the screened powder was then conventionally enclosed within a can of soft steel, for example, and the can was Hot Isostatically Pressed or HIPed to consolidate the powder into a useful form. Such HIPing may be followed by extruding or other conventional processing steps to bring the consolidated product to a usable form.
An alternative to the powder metallurgy processing as described immediately above is a conventional process known as spray forming. Spray forming has been described in a number of patents including the U.S. Pat. Nos. 3,909,921; 3,826,301; 4,926,923; 4,779,802; 5,004,153; as well as a number of other such patents.
Spray forming is a process using gas atomization to make a spray of droplets of liquid metal followed by solidification of the spray on a solid body to directly form a billet or billet preform. This process was originally developed by Osprey Metals, Ltd.
In general, the spray forming process has been gaining additional industrial use as improvements have been made in processing, particularly because it involves fewer steps and has a cost advantage over conventional powder metallurgy techniques so there is a tendency toward the use of the spray forming process where it yields products which are comparable and competitive with the products of the conventional powder metallurgy processing. An unavoidable byproduct of spray forming is overspray, which is the metal that solidifies in flight, without attaching to the preform. This overspray has in the past been collected in powder form and has been remeited or HIPed for commercial use.
Since the overspray results in some inefficiencies in the spray forming operations performed in the direct processing of electroslag refined metal, a method for recycling such overspray and reprocessing it directly into the electroslag refining apparatus would be desirable as opposed to having it remelted or HIPed in a separate process. Such method should provide for the injection of the overspray powder directly back into the electroslag refining apparatus, such as for example, onto the top of the slag in the ESR furnace where the powder would melt, drop through the slag, and pour through the CIG nozzle. Such method should also be relatively simple, inexpensive and easy to implement as well as possibly resulting in significant cost savings.